The present invention relates to a new process for preparing N,N-disubstituted p-phenylenediamine derivatives of formula I ##STR3## wherein R.sup.1 is alkyl of 1 to 6 C atoms, R.sup.2 is alkyl of 1 to 6 C atoms or alkyl of 1 to 6 C atoms which is substituted by OH, lower alkoxy, a sulfo group or an alkylsulfonamido group, and R.sup.3 is hydrogen or lower alkyl, and acid addition salts thereof, by nitrosation of the corresponding aniline derivatives and subsequent hydrogenation of the N,N-disubstituted p-nitroso-aniline derivatives formed.
Such N,N-disubstituted p-phenylenediamine derivatives have been known for a long time (compare, for example, J.Amer.Chem.Soc. 73, 3,100, 1951). They have achieved great importance, in particular, as developer substances for color photography and also as intermediate products in the preparation of a very wide range of azo dyestuffs. Because the importance of these substances, which are at present already required in large amounts, is increasing further and the demand for them is rising, a number of processes, and in particular improved process conditions, are already known for the preparation of these compounds. However, in all the known processes for preparing N,N-disubstituted p-phenylenediamine derivatives, sodium nitrite is used for the nitrosation of the corresponding aniline derivatives. This nitrosation is carried out in aqueous or aqueous-alcoholic solution. Adding the nitrous acid required for the nitrosation in the form of its sodium salt requires, however, that the sodium added must be separated off again in the form of an inorganic salt after the nitrosation or, at the latest, after the subsequent hydrogenation to give the corresponding p-phenylenediamine derivatives. Furthermore, a number of undesired, troublesome by-products and, in particular secondary products are formed in this procedure. It is therefore necessary for the organic intermediate products and end products to be intermediately isolated and purified, even if, as proposed in a review (J.Amer.Chem.Soc. 73, 3,100, 1951), the reaction mixture containing the p-nitroso-aniline derivative is employed directly in the subsequent hydrogenation. Moreover, the intermediate presence of the unstable free p-phenylenediamine bases, which in some cases are allergenic, cannot be avoided in this procedure.
Although the problems occurring in the preparation of N,N-disubstituted p-phenylenediamine derivatives, that is to say the formation of undesired, troublesome by-products and secondary products, losses in yield, inadequate purity of the end products and the handling of unstable intermediate products, which in some cases are allergenic, have been recognized, a successful solution to these problems has not yet been proposed anywhere.